1. Field of the Invention
The present invention generally relates to an optical scanning device used in a writing optical system of an image forming apparatus such as a laser printer, a digital copier, a facsimile machine or the like, a measurement apparatus, an inspection apparatus or the like, and to a line-image forming optical system used in an optical scanning device, an imaging adjustment method used in the optical scanning device and an image forming apparatus.
2. Description of the Related Art
An optical scanning device deflecting a light flux from a light source, condensing the light flux onto a surface to be scanned as a beam spot through a scanning optical system and scanning the surface to be scanned has been well-known in connection with a laser printer, a digital copier, a facsimile machine, and so forth. In such an optical scanning device, in order to reduce a lens cost and/or in order to achieve a special lens surface shape, a resin-made lens (meaning a lens made of resin, through the specification and claims) is used. Especially, various shapes of lens surface have been proposed for a scanning and imaging lens of a scanning and imaging optical system causing a deflected light flux to form an image on a surface to be scanned, in order to well correct curvature of field and uniform-velocity characteristics such as a linearity, and, a resin-made lens is suitable for achieving such a special lens surface shape.
However, as well-known, a resin-made lens involves a problem in that, a change in volume due to a change in temperature causes a curvature and/or refractive index of the lens to change, and a lens performance, in particular, a position of focus on a surface to be scanned to change. Such a change in position of focus results in increase of a spot diameter of a beam spot on a surface to be scanned, and degradation of resolution in optical scanning.
In this regard, because a change in position of focus due to a change in temperature of resin-made lenses occurs in a positive lens and a negative lens reversely to one another, it has been proposed to cancel a change in position of focus of a resin-made scanning and imaging lens due to a change in temperature by disposing a resin-made lens having a power reverse to that of the resin-made scanning and imaging lens on a light path from a light source to a light deflector (see Japanese Laid-Open Patent Application No. 8-160330 and Japanese Laid-Open Patent Application No. 8-292388).
An optical scanning device disclosed in Japanese Laid-Open Patent Application No. 8-160330 includes a light source, a deflector, an entrance optical system, a scanning optical system, and a medium to be scanned. The entrance optical system includes a first optical system (collimator lens) transforming a divergent light flux from the light source into a parallel light flux, and a second optical system condensing the light flux from the first optical system in sub-scanning direction so as to cause the light flux to form an image on or in the proximity of the deflector. Either one of the first and second optical system includes a resin-made optical component (lens) having a negative power in sub-scanning direction.
A scanning optical device disclosed in Japanese Laid-Open Patent Application No. 8-292388 has a negative lens having a negative refracting power only in sub-scanning direction and made of resin in a first imaging part forming an image on or in the proximity of a deflection position of a deflector, and performs temperature compensation.
However, in such an optical scanning device, a resin-made lens disposed between the light source and deflector for the correction has a negative power only in sub-scanning direction and no power in main scanning direction. Accordingly, it is not possible to correct a shift in focus position (shift in imaging position of a beam spot) in main scanning direction due to a change in temperature of a scanning and imaging lens.
Further, a lens for the correction in the related art has an ordinary arc-shape lens section. Accordingly, wavefront aberration is increased by the lens for the correction, and, thereby, achievement of a small-diameter beam spot is obstructed.
An optical scanning device xe2x80x98transforms a beam (denoting an xe2x80x98optical beamxe2x80x99 through the specification and claims) from a light source into a beam having a predetermined beam style by a coupling optical system, causes this beam to form a line image longin main scanning direction by a line-image forming optical system, deflects the beam by a light deflector having a deflection reflective surface at or in the proximity of the imaging position of this line image, condenses the deflected beam toward a surface to be scanned by a scanning and imaging optical system, forms a beam spot on the surface to be scanned, and scans the surface to be scannedxe2x80x99. By thus causing a beam from the light source to form a line image long in main scanning direction on or in the proximity of the deflection reflective surface of the light deflectorxe2x80x99, it is possible to correct so-called xe2x80x98surface inclinationxe2x80x99 of the light deflector.
Recently, increase in image density has been demanded in a digital copier and/or a laser printer, and reduction in diameter of beam spot formed on a surface to be scanned is demanded. Further, as mentioned above, making a lens from resin in the scanning and imaging optical system has been performed progressively in order to reduce costs and/or to achieve xe2x80x98special surface shapexe2x80x99 needed to reduce diameter of beam spot.
A beam spot is formed as a result of a deflected beam deflected by the light deflector being condensed toward the surface to be scanned by the scanning and imaging optical system. Ideally, the beam spot is formed by a beam waist of the condensed deflected beam. In the optical scanning device having the above-described configuration, the scanning and imaging optical system is an xe2x80x98anamorphic optical system such that power thereof is different between main scanning direction and sub-scanning directionxe2x80x99. Accordingly, generally, the position of beam waist of the deflected beam in main scanning direction does not completely coincide with the position of beam waist of the deflected beam in sub-scanning direction.
An image surface in main scanning direction and sub-scanning direction is obtained from a collection the beam-waist positions in main scanning direction and sub-scanning direction. The image surface is curved in accordance with curvature of field of the optical system.
In order to achieve satisfactory optical scanning with xe2x80x98a beam spot having a diameter small in both main scanning direction and sub-scanning directionxe2x80x99, it is necessary that the beam-waist positions in main scanning direction and sub-scanning direction substantially coincide with the surface to be scanned xe2x80x98whatever the position in main scanning direction (image height)xe2x80x99, that is, the image surface in main scanning direction and sub-scanning direction substantially coincides with the surface to be scanned. In order to satisfy this condition, optical design is made so as to well correct curvature of field in main scanning direction and sub-scanning direction.
However, although a satisfactory result is obtained on design, optical characteristics on design are not actually achieved. For example, when an fxcex8 lens which is generally used as the scanning and imaging optical system includes working errors and/or assembly errors, an actual image-surface position of a deflected beam is different from the surface to be scanned. As a result, the diameter of the beam spot is larger than the designed amount.
It is possible to form a beam spot substantially having a spot diameter in accordance with design on the surface to be scanned by making an fxcex8 lens or the like xe2x80x98substantially in accordance with designxe2x80x99 and assembling it with high accuracy. However, when the fxcex8 lens includes a xe2x80x98resin-made lensxe2x80x99, a change in refractive index and/or a change in shape of the resin-made lensxe2x80x99 occurs when the temperature of the optical scanning device changes, and, thereby, a xe2x80x98shift of image surfacexe2x80x99 in that the image surface shifts from the surface to be scanned occurs, and the spot diameter increases.
Further, when a metal holding device holding the coupling optical system expands or shrinks due to that temperature change, a minute distance between the light source and coupling optical system changes, and, thereby, the coupling function changes, and the spot diameter increases.
A direction of shift of image surface (direction in which the image surface moves) due to xe2x80x98temperature changexe2x80x99 of a resin-made lens is reverse for the lens having a positive power to that for the lens having a negative power. Accordingly, by providing a xe2x80x98resin-made lens having a power reverse to that of a resin-made lens of fxcex8 lensxe2x80x99 on a light path from the light source up to the light deflector, and adjusting the powers of the lenses, it is possible to cancel the shift of image surface due to temperature change (as disclosed in Japanese Laid-Open Patent Application No. 8-292388 and Japanese Patent No. 2804647).
As mentioned above, in the method disclosed in Japanese Laid-Open Patent Application No. 8-292388, a resin-made lens disposed between a light source and a light deflector xe2x80x98does not have power in main scanning directionxe2x80x99. Accordingly, this lens does not have a function of correcting of shift of image surface in main scanning direction due to temperature change. Therefore, it is not possible to prevent a spot diameter from increasing in main scanning direction.
Further, in all the embodiments disclosed in Japanese Laid-Open Patent Application No. 8-292388, a xe2x80x98resin-made lens having a negative powerxe2x80x99 is a cylindrical plane-concave lens. However, the radius of curvature thereof is 5 through 8 mm, that is, very small. Therefore, high accuracy is demanded for working and assembly of the lens. This is because the function of correction coping with temperature change is performed by xe2x80x98only one surfacexe2x80x99 of the lens.
According to the Japanese Patent No. 2804647, a resin-made lens having power xe2x80x98having an absolute value equal to and a sign reverse toxe2x80x99 those of power of a resin-made lens included in a scanning and imaging lens is used and shift of image surface in main scanning direction is corrected, and, the position of the scanning and imaging optical system is controlled and shift of image surface in sub-scanning direction is controlled to a level such as not to cause problems. By this method, it is possible to effectively reduce xe2x80x98shift of image surface due to temperature changexe2x80x99 in both main scanning direction and sub-scanning directionxe2x80x99. However, an arrangement of the scanning and imaging optical system is limited. Accordingly, flexibility in optical design of the optical scanning device is remarkably limited.
Further, Japanese Laid-Open Patent Application No. 10-20225 and Japanese Patent No. 2761723 disclose a method of mechanically moving collimator lens and so forth in optical-axis direction, and correcting shift of image surface. However, in this method, extra mechanical components, xe2x80x98components for detecting that the imaging position is shiftedxe2x80x99 and so forth are needed. Therefore, costs increase, and power consumption increases.
The present invention has been devised in consideration of the above-described circumstances, and an object of the present invention is to provide an optical scanning device which can form a small-diameter beam spot on a surface to be scanned whatever temperature change, by self correcting shift of focal position in main scanning direction and sub-scanning direction due to change in ambient temperature through all the optical system, even when a resin-made lens is used in an optical system causing a deflected beam to form an image on the surface to be scanned.
Another object of the present invention is to make it possible to correct shift of image surface due to error in working and/or error in assembly of lens in main scanning direction and sub-scanning direction independently.
Another object of the present invention is to effectively reduce shift of image surface due to temperature change in main scanning direction and sub-scanning direction, without using extra mechanical components, with resin-made optical component which is easy to work and/or assemble, without reducing flexibility of design.
An optical scanning device according to the present invention comprises:
a light source emitting a light flux;
a coupling optical system coupling the light flux from the light source to a subsequent optical system by transforming it into a parallel light flux, an approximately convergent light flux or an approximately divergent light flux;
a light deflector reflecting the light flux from the coupling optical system with a deflection reflective surface, and deflecting it;
a scanning and imaging optical system condensing the deflected light flux from the light deflector onto a surface to be scanned as a beam spot; and
a correcting optical system for self correcting shift of focal position of the beam spot on the surface to be scanned occurring due to environmental change or the like,
wherein the correcting optical system comprises at least one pair of a resin-made lens having an anamorphic surface having a negative power in each of main scanning direction and sub-scanning direction and a glass-made lens having an anamorphic surface having a positive power at least in sub-scanning direction, and is disposed between the coupling optical system and deflection reflective surface.
Thereby, by suitably determining the temperature dependency of curvature, coefficient of linear expansion, and refractive index of the anamorphic surface of each of the lenses of the correcting optical system for correcting shift of imaging position in main scanning direction and shift of imaging position in sub-scanning direction on the surface to be scanned due to environmental change, it is possible to correct shift of focal position occurring due to change of environment (temperature, humidity) for both main scanning direction and sub-scanning direction. Further, by employing the anamorphic surface, it is possible for each lens to have a plane surface on the reverse side. This plane surface can be used as a reference surface for mounting the lens. Accordingly, it is possible to prevent eccentricity which degrades optical performance from occurring.
It is preferable that the correcting optical system comprising the resin-made lens having the anamorphic surface and glass-made lens having the anamorphic surface is integrally held by a holding member.
Thereby, it is possible to control, previously at a time of assembling adjustment, wavefront aberration occurring in the correcting optical system
It is preferable that a configuration is provided such that the correcting optical system integrated by the holding member can be moved in direction of optical axis.
Thereby, when assembling the correcting optical system in an optical system unit, it is possible to perform adjustment such as to prevent shift of focal position previously.
It is preferable that a configuration is provided such that the correcting optical system integrated by the holding member can be rotated about optical axis.
Thereby, it is possible to adjust the entirety of the optical system while keeping the wavefront aberration in the correcting optical system smaller.
It is preferable that at least one surface of the anamorphic surfaces of the correcting optical system comprises an aspherical surface in each of main and sub-scanning directions.
Thereby, it is possible to well correct the wavefront aberration.
It is preferable that, when L denotes a surface separation between the resin-made lens having the anamorphic surface and the glass-made lens having the anamorphic surface and fs denotes a focal length of the entirety of the correcting optical system, the following condition is satisfied:
0 less than L/fs less than 0.1 
Thereby, it is possible to well correct the wavefront aberration and to obtain a small-diameter beam spot.
An imaging adjustment method in an optical scanning device according to the present invention is a method of performing adjustment of imaging of deflected beam in an optical scanning device through which a beam in a predetermined style from the side of a light source is caused to form a line image long in main scanning direction by a line-image forming optical system, then, is deflected by a light deflector having a deflection reflective surface on or in the proximity of the imaging position of the line image, the deflected beam is condensed toward a surface to be scanned by a scanning and imaging optical system, a beam spot is formed on the surface to be scanned, and optical scanning of the surface to be scanned is performed, and has the following features:
The line-image forming optical system is configured to have a plurality of optical components, these optical components are configured to include a plurality of surfaces each having a negative power in sub-scanning direction and a surface having a negative power in main scanning direction.
The positional relationship of the optical component having the surface(s) having the negative power in main scanning direction and the other optical component having the surface(s) having the negative power in sub-scanning direction with the light-source-side optical system (light source and coupling optical system) and/or scanning and imaging optical system is adjusted independently for the above-mentioned optical component and the above-mentioned other optical component. Thereby, the beam-waist positions in main scanning direction and sub-scanning direction are adjusted with respect to the position or the surface to be scanned.
The relative positional relationship between the above-mentioned optical component having the surface(s) having the negative power in main scanning direction and the light-source-side optical system and/or scanning and imaging optical system is adjusted, as mentioned above. Thereby, it is possible to adjust the position of image surface in main scanning direction with respect to the surface to be scanned. Further, the relative positional relationship between the above-mentioned other optical component having the surface(s) having the negative power in sub-scanning direction and the light-source-side optical system and/or scanning and imaging optical system is adjusted, as mentioned above. Thereby, it is possible to adjust the position of image surface in sub-scanning direction with respect to the surface to be scanned.
This imaging adjustment method is a method of adjusting so as to correct shift of image surface occurring due to working errors and/or assembly errors. Accordingly, this method can be performed whether or not the line-image forming optical system and/or scanning and imaging optical system includes resin-made optical components. When optical components of the line-image forming optical system are to be moved in the relative positional adjustment of the above-mentioned optical component and/or the above-mentioned other optical component, it is not necessary to use a special mechanism. It is sufficient that a suitable jig is used and the movement is made, and, then, after the adjustment, the optical components are fixed using adhesive or the like.
A line-image forming optical system according to the present invention is a line-image forming optical system in an optical scanning device through which a beam from a light source is transformed into a beam in a predetermined style by a coupling optical system, the beam is caused to form a line image long in main scanning direction by the line-image forming optical system, then, is deflected by a light deflector having a deflection reflective surface on or in the proximity of the imaging position of the line image, the deflected beam is condensed toward a surface to be scanned by a scanning and imaging optical system, a beam spot is formed on said surface to be scanned, and optical scanning of said surface to be scanned is performed.
The line-image forming optical system comprises at least one resin-made lens and at least one glass-made lens;
the at least one resin-made lens includes at least two surfaces each having a negative power in sub-scanning direction and at least one surface each having a negative power in main scanning direction; and
the powers of respective surfaces of the at least one resin-made lens are set so that shift of image surface occurring due to temperature change of the coupling optical system and/or scanning and imaging optical system is effectively reduced.
Alternatively, the above-mentioned line-image forming optical system comprises at least one resin-made imaging mirror and at least one glass-made lens;
the at least one resin-made imaging mirror has at least one surface having a negative power in main scanning direction and a larger negative power in sub-scanning direction; and
the powers of the surface of the at least one resin-made imaging mirror are set so that shift of image surface occurring due to temperature change of the coupling optical system and/or scanning and imaging optical system is effectively reduced.
Alternatively, the above-mentioned comprises at least one resin-made imaging mirror, at least one resin-made lens, and at least one glass-made lens;
the at least one resin-made imaging mirror has at least one surface having a negative power at least in sub-scanning direction;
the at least one resin-made lens has at least one surface having a negative power at least in sub-scanning direction;
a system consisting of these resin-made imaging mirror and resin-made lens has at least one surface having a negative power in main scanning direction; and
the powers of respective surfaces of at least one resin-made imaging mirror and at least one resin-made lens are set so that shift of image surface occurring due to temperature change of the coupling optical system and/or scanning and imaging optical system is effectively reduced.
Thus, each of the these line-image forming optical systems effectively reduces shift of image surface occurring due to temperature change of the coupling optical system and/or scanning and imaging optical system. The shift of image surface due to temperature of the coupling optical system is shift of image surface occurring due to a slight change of the distance between the coupling optical system and light source due to expansion or shrinkage of a metal holder holding the coupling optical system and/or a change of optical characteristics due to temperature change of a resin-made optical component when the coupling optical system includes the resin-made optical component as described above. This shift of image surface may occur when the scanning and imaging optical system does not include optical components made of resin.
The shift of image surface due to temperature change of the scanning and imaging optical system is shift of image surface occurring due to a change of optical characteristics due to temperature change of a resin-made optical component when the scanning and imaging optical system includes the resin-made optical component
When the scanning and imaging optical system includes a resin-made optical component, the line-image forming optical system may comprise two resin-made lenses and a glass-made lens;
the two resin-made lenses including three surfaces each having a negative power in sub-scanning direction, and a surface having a negative power in main scanning direction; and
the powers of respective surfaces of the two resin-made lenses being set so that shift of image surface occurring due to temperature change of the resin-made optical component included in the scanning and imaging optical system is effectively reduced.
In each of the above-described line-image forming optical systems, the powers of the surfaces having the negative powers in sub-scanning direction included in the resin-made optical components (resin-made imaging mirror and/or resin-made lens) may be set to be approximately equal to each other.
Each of the above-mentioned line-image forming optical system can effectively reduce for main and sub-scanning directions shift of image surface occurring due to temperature change by canceling it out by shift of image surface occurring due to change of characteristics due to temperature change of resin-made optical components.
As described above, a large negative power in sub-scanning direction is needed for correcting shift of image surface in sub-scanning direction. However, by configuring the resin-made lens(es) to include the plurality of surfaces each having the negative power in sub-scanning direction, it is possible to distribute the negative power in sub-scanning direction required for correcting the shift of image surface among the plurality of lens surfaces. Thereby, it is possible to reduce the negative power of each lens surface, to effectively prevent the radius of curvature of the lens surface in sub-scanning direction from being so reduced, and to ease manufacture and assembly of the resin-made lens.
Further, when the resin-made imaging mirror is used, because the negative power of resin-made imaging mirror can be achieved by a curvature smaller in comparison to a case where refraction in a lens surface is used. Accordingly, a radius of curvature of the resin-made imaging mirror may be enlarged. Therefore, it is possible to achieve correction of shift of image surface only by a single resin-made imaging mirror, while manufacture and assembly of the resin-made imaging mirror is easy.
An optical scanning device according to the present invention is an optical scanning device through which a beam in a predetermined style from a side of a light source is caused to form a line image long in main scanning direction by a line-image forming optical system, then, is deflected by a light deflector having a deflection reflective surface on or in the proximity of the imaging position of the line image, the deflected beam is condensed toward a surface to be scanned by a scanning and imaging optical system, a beam spot is formed on the surface to be scanned, and optical scanning of the surface to be scanned is performed.
In the optical scanning device, any of the above described line-image forming optical systems can be used as the line-image forming optical system of the optical scanning device.
In the above-description, any one of various gas lasers, solid lasers, semiconductor lasers, LEDs can be used as the light source. Further, the optical scanning device may be not only of a so-called single-beam type but also of a multi-beam type. In this case, a beam combining type light source device performing beam combination of beams from semiconductor laser array or a plurality of light sources by a prism can be used as the light source of the optical scanning device. Further, a rotational single-surface mirror, rotational bi-surface mirror, a rotational polygon mirror, or a galvano-mirror may be used as the light deflector having the deflection reflective surface.
An image forming apparatus according to the present invention is an xe2x80x98image forming apparatus forming an electrostatic latent image on a latent-image carrying body through optical scanning, visualizing the formed electrostatic latent image, and obtaining a desired recorded imagexe2x80x99.
Any of the above-described optical scanning devices may be used as an optical scanning device performing the optical scanning of the latent-image carrying body of the image forming apparatus according to the present invention.
In this case, a photoconductive photosensitive body may be used as the latent-image carrying body, the electrostatic latent image may be formed on a photosensitive surface thereof through uniform charging and the optical scanning thereof, and the thus-formed electrostatic latent image may be visualized as a toner image. The toner image is fixed onto a sheet-like recording medium (transfer paper, plastic sheet for an overhead projector(OHP sheet), or the like).
In the image forming apparatus, a film for photography with silver halide may be used as the image carrying body, for example. In this case, the electrostatic latent image formed through the optical scanning by the optical scanning device is visualized by a xe2x80x98method of developing in an ordinary process of photography with silver halidexe2x80x99. Such an image forming apparatus may be embodied as an optical plate-making system, an optical drawing system, or the like.
The image forming apparatus according to the present invention may also be embodied as a laser printer, a laser plotter, a digital copier, a facsimile machine, or the like.
Thus, as described above, according to the present invention, novel optical scanning device, line-image forming optical system in the optical scanning device, imaging adjustment method for the optical scanning device and image forming apparatus can be achieved.
By using a line-image forming optical system according to the present invention, it is possible to effectively reduce for main and sub-scanning directions shift of image surface occurring due to temperature change of optical components made of resin included in the optical scanning device, without using an extra mechanical configuration, without reducing flexibility of design, using resin-made optical components which are easy to manufacture and assemble. Accordingly, an optical scanning device using this line-image forming optical system can effectively prevent a spot diameter from increasing and to achieve satisfactory optical scanning, whatever the temperature change, within a practical range of temperature change.
Further, by the imaging adjustment method according to the present invention, it is possible to adjust the beam-waist positions in main scanning direction and sub-scanning direction of the deflected beam, which may shift due to working errors of optical component such as fxcex8 lens or the like, mutually independently for the main and sub-scanning directions with respect to the surface to be scanned. Then, when the optical components of the line-image forming optical system are fixed after the adjustment, the beam-waist positions in main and sub-scanning directions well coincide with the surface to be scanned. Thereby, it is possible to achieve satisfactory optical scanning.
Further, an image forming apparatus according to the present invention can achieve satisfactory image formation whatever temperature change by using the above-mentioned optical scanning device.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.